[Mojo] Almost connect mojo_js with hello_world_service

This CL connects mojo_js with hello_world_service. After this
CL, the JavaScript and C++ implementations have reached
parity.

BUG=317398

Review URL: https://codereview.chromium.org/82953004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237018 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237018